In the field of internal combustion engines, turbochargers are forced-induction devices that are utilized to increase the pressure of the intake air provided to the engine. Exhaust gases from the engine are routed to the turbocharger and are utilized to drive a turbine wheel. The rotational force generated by the turbine wheel is utilized to drive a compressor wheel, which pressurizes ambient intake air and supplies the pressurized intake air to the engine. By pressurizing the intake air, the amount of air and fuel that can be forced into each cylinder during an intake stroke of the engine is increased. This produces an increased power output relative to a naturally-aspirated engine.
The turbine wheel is connected to the compressor wheel by a shaft, with the turbine wheel being joined to a first end of the shaft and the compressor wheel being connected to a second end of the shaft. The shaft passes through a bearing housing and is supported by a plurality of bearings that allow the shaft to spin with respect to the bearing housing at extremely high rotational speeds. The bearings are lubricated by oil, for example, via an oil line that delivers pressurized engine oil to the bearing housing from the oil pump of the engine.
At each end of the bearing housing, the shaft exits the bearing housing and enters either the turbine housing or the compressor housing. Piston rings are utilized at each end of the housing to prevent oil from exiting the bearing housing along the shaft. In particular, the piston rings are seated against the bearing housing or a component within the bearing housing in a manner that restrains rotation of the piston rings with respect to the bearing housing. The piston rings extend into respective grooves that are formed on the shaft.
The grooves on the shaft are formed such that they are wider than the piston rings. This axial clearance between the piston ring and the groove helps to prevent excessive contact between the piston ring and the groove. The axial clearance is typically between 20 microns and 100 microns.
The term “blow by” refers to oil that travels past the piston rings. Axial clearance is a primary controlling factor for blow by for a given pressure differential across a piston ring. As axial clearance increases, blow by increases.